ScrollablePage: Port sizing to more conventional expressions and ?. operator
Modern JS nullish-coalescing and optional chaining operators let us write shorter and subjectively more readable code.
Regarding sizing, they were both far from conventional bindings. The implicitHeight got ported to relatively new-ish all-in-one implicitHeaderWidth/implicitFooterWidth properties, which it couldn't use before because of import version restrictions. The implicitWidth just omitted a lot of components for seemingly no reason.
I wonder if contentHeight should fallback to undefined
to reset it
(QQuickPane supports that).